1. Field of the Invention
The present invention relates to data storage libraries, which transport and access media items using media accessors. More particularly, the invention concerns an apparatus, method, and article of manufacture for non-invasive recalibration of an accessor in a data storage library.
2. Description of the Related Art
Data storage libraries are one of the most popular types of mass storage system today. Generally, a data storage library connotes a great number of co-located portable data storage media, transported by a robotic mechanism between locations such as storage cells, read/write media drives, and input/output facilities.
These libraries have become popular for many reasons. First, the portable data storage media, usually magnetic tape or optical media, are typically inexpensive relative to other storage formats such as magnetic disk drives. Furthermore, libraries are easily expanded to accommodate more data, simply by adding more items of media and storage cells. Additionally, most libraries can be easily updated with new equipment as it comes onto the market. For example, a new model tape drive may be introduced to the library to supplement or replace the existing drives.
A typical library includes an expandable protective structure, constructed of various doors and panels, to house the library's drives, storage cells, robotic device, and related electronic equipment. The robotic device, called an "accessor" includes appropriate mechanical and electrical subcomponents to move side-to-side, lengthwise, and up-down within the protective structure. Accessors also include features needed to grasp and release media items. Importantly, the library must also include an alignment system for the accessor. Such a system typically includes a light source aboard the accessor, and multiple light-absorbing or light-reflecting "fiducials" strategically located throughout the library.
To best use the fiducials, one or more are usually collocated with each removable component of the library, such as a drive or panel of storage cells. This way, the accessor can easily relocate a new, moved, or replaced component by finding its corresponding fiducial. The process of training the accessor's electronics to find and remember a fiducial's location is called "teaching" the accessor.
Besides the addition, movement, or replacement of library components, teaching is also necessary when the accessor itself is removed or adjusted; in this case, the accessor must be sequentially advanced to each fiducial's location. More particularly, when the accessor is removed or adjusted, at least one conventional teaching technique operates as follows.
For each fiducial, this technique retrieves the fiducial's position from a storage location. Then, the accessor moves to that fiducial and determines the fiducial's position, for example in an x-y coordinate system. A difference value is computed between this position and the same fiducial's previously stored position, and this difference value stored in association with each cell, drive, or other component whose position is linked to that fiducial. The entire process is repeated for each fiducial in the system. After the teaching ends, the accessor operates using the stored positions of the cells and drives as updated by their newly computed difference values.
Despite its usefulness and reliability, the foregoing teaching process still occurs at the expense of customer "jobs", since the library is taken offline to "teach" the accessor. In some libraries, reteaching the accessor can take as little as fifteen minutes to more than one hour, depending on the library's size. Furthermore, this downtime can be especially frustrating in systems utilizing a second accessor to improve media access time, because the second accessor still must be disabled during teaching of the first accessor.
Design engineers are frustrated by any unavailability of the library for customer jobs because such work is the underlying reason for the library in the first place. Plus, many applications can be especially intolerant of library "down-time". For example, continuous data availability is critical for telephone companies, stock brokerages, companies with twenty-four hour sales lines, companies providing information internationally to those in many different time zones, etc.
Consequently, certain libraries may not be completely adequate for some applications due to these conditions.